VOLUME 2 Post-Treatment, Reuse, and Disposal
4.3 Components of Municipal Water Demand
Municipal water demand in the United States varies from 300 to1800 liters per capita per day (Lpcd).1,5–7 Various components of municipal water uses are: (1) residential or domestic, (2) commercial, (3) indus- trial, (4) institutional and public, and (5) water lost or unaccounted for.1
The relative proportions of these uses with respect to overall municipal water demand are summarized inTable 4.1. General discussion is presented below.
4.3.1 Residential or Domestic Water Use
The residential or domestic water uses include toiletflush, bathing and washing, cooking and drinking, lawn watering, and others. Typical breakdown of residential water uses andflow rates are summarized inTable 4.2. The average residential water demand varies from 300 to 450 Lpcd. Withflow reduction devices, the residential water demand for many types of residential establishments is decreasing.
4.3.2 Commercial Water Use
Commercial establishments include motels, hotels, office buildings, shopping centers, service stations, movie houses, airports, and the like. The water uses in commercial establishments vary greatly depending upon the size of operation. The commercial water demand may be estimated from unit loading orfloor area. Typical unit loadings of residential and commercial establishments are provided inTable 4.3.
4.3.3 Institutional and Public Water Use
Water uses in public buildings and institutional establishments (city halls, prisons, hospitals, schools, etc.) as well as water used for public services (fire protection, street washing, park irrigation, and the like) are considered public water uses. Some residential, commercial, and institutional water uses are provided inTable 4.3.
TABLE 4.2 Typical Breakdown and Flow Rates of Residential Water Uses Types of Water Use Non Conserving
Home Usage, % Flow Rate
Toiletflush, including toilet leakage 33 Tank type 19–27 L=use Valve type 90–110 L=min Shower and bathing
Shower 20 Shower head 90–110 L=use or 19–40 L=min
Bathing 8 Tub bath 60–90 L=use
Wash basin 11 4–8 L=use
Kitchen 9
Drinking, cooking 2–6 Kitchen sink
Dishwashing 3–5 15–30 L=dishwasher load
Garbage disposal 0–6 6000–7500 L=week, 4–8 L=person·d
Laundry and washing machine 16 110–200 L=load
Lawn 3 Sprinkler system
Source:Adapted in part from References 1, 2, 4, and 8.
TABLE 4.1
Components of Municipal Water Demand Percent of Total
Residential or domestic 35–50
Commercial 10–25
Industrial 15–30
Institutional and public 5–20
Water lost or unaccounted for 5–10
Source:Adapted in part from References 1, 2, 4, and 6.
TABLE 4.3 Average Water Demand in Residential, Institutional, Commercial, and Industrial Establishments
Source Unit Unit Flow, m3=unit·d
Residential Single-family detached, low income to
high income
Person 0.25–0.38
Apartment Person 0.18–0.23
Trailer park Person 0.15
Commercial Country club
Resident Member 0.38
Nonresident Member 0.10
Hotel=Motel Unit guest 0.38
Resort Unit guest 0.19
Restaurant Customer 0.03
Bar Customer 0.08
Store Toilet room 1.52
Employee 0.04
Department store and shopping center Employee 0.04
Per m2floor area 0.001–0.002
Office building and complex Employee 0.065
Per m2floor area 0.015
Movie Seat 0.008
Laundromat Machine 2.5
Barber shop Chair 0.065
Beauty salons Station 1.026
Service station First bay 3.8
Additional bays 1.9
Industrial
Industrial building Employee 0.055
Factories
With shower Employee-shift 0.133
Without shower Employee-shift 0.095
Light industrial zone ha 9–14
Medium industrial zone ha 14–30
Heavy industrial zone ha 30–100
Industrial products
Cattle Head 0.04–0.05
Dairy Head 0.07–0.08
Chicken Head 0.03–0.04
Canning Metric ton=d 30–60
Dairy, milk Metric ton=d 2–3
Meat packing Metric ton=d 15–25
Pulp and paper Metric ton=d 200–800
Steel Metric ton=d 260–300
(Continued)
4.3.4 Industrial Water Use
Industrial water demands in the United States are very large. Generally, large industries develop their own water supply systems. Only small industries purchase water and therefore impose demand on local municipal system. The industrial water demand may be estimated on the basis of proposed industrial zoning and unit loadings for specific industries. Unit loadings for many industries are summarized in Table 4.3.
4.3.5 Water Unaccounted for or Lost
Major sources of water unaccounted for are leaks from mains, faulty meters, and unauthorized connec- tions. This loss mostly depends upon the condition of water distribution system. In many developing nations where individual water connections are unmetered, as much as 30–50% water supplied may be lost or unaccounted for due to old and leaky distribution system and wasted from unmetered connec- tions. To reduce these losses, water is supplied only for a few hours in the mornings and in the evenings.
This practice does save water but may cause serious contamination of water supply from the entry of polluted surface and groundwater into the distribution system during the periods when the supply is interrupted.
4.3.6 Factors Affecting Water Use
The water usage in a community depends upon many factors. Many of these factors are:
1. Geographical location and climate:More water is used in hot and dry climate or region than that in wet, humid, and cold climate or region.
2. Size, population density, and economic conditions of the community: Wealthier and sparsely populated communities have higher water demand.
3. Industrialization: Industries use large amounts of water especially seasonal industries such as vegetable canneries and others.
4. Metered water supply and cost:Less water is used where water supply is individually metered. Also, water usage is closely related to cost of water.
5. Water pressure:Higher supply pressure may cause unnecessary waste due to spurts from faucets, leaks, and drips.
TABLE 4.3 (Continued) Establishments
Source Unit Unit Flow, m3=unit·d
Tannery Metric ton of raw hides
processed=d 60–70
Institutional and Public Uses
Nursing homes Bed 0.38
Hospital Bed 0.95
Prison Inmate 0.45
School
Boarding Student 0.3
Day Student 0.076
Note: m3264.5¼gallon
Source: Adapted in part from References, 1, 2, 4, 6, and 8 through 10.
6. Water conservation:Efforts of general public and regulatory agencies in water conservation have resulted in 20–30% savings. These savings are achieved by pressure reducing valves, faucet aerators,flow limiting shower heads, shallow trap water closet, level-controlled washing machines, and education of consumers.1–3,7–9
EXAMPLE 4.3: MUNICIPAL WATER DEMAND AND COMPONENTS
A city has a population of 30,000 residents. The residential water demand is 210 Lpcd. The commercial, industrial, institutional and public water uses, and water lost or unaccounted for are 18%, 23%, 10%, and 8%, respectively, of total municipal water demand. Calculate total municipal water demand in m3/d and MGD, and the demand for each component.
Solution
1. Calculate the residential water demand.
Residential water demand=30,000 residents× 210 L
person·d× m3
1000 L=6300 m3/d or 1.66 MGD 2. Calculate the total municipal water demand.
Total demand for commercial, industrial, institutional and public, and water lost or unaccounted for
¼(18þ23þ10þ8)%
¼59% of total municipal water demand Residential water demand ¼(10059)%¼41% of municipal water demand Total municipal water demand ¼ 6300 m3=d residential
0:41 residential=total municipal¼15,366 m3=d 3. Calculate the water demand for each component.
Water demand for each component is tabulated below:
Water Use Percent of Total Water Demand m3=d MGD
Residential 41 6300 1.66
Commercial 18 2766 0.73
Industrial 23 3534 0.93
Institutional and public use 10 1537 0.40
Water lost or unaccounted for 8 1229 0.32
Total 100 15,366 4.04
EXAMPLE 4.4: WATER DEMAND AND WASTEWATER FLOW
A subdivision of a suburban city is being developed. The ultimate zoning plan shows the following res- idential, commercial, industrial, and institutional establishments. Assume that the water lost is small and is ignorable. Using the average values given inTable 4.3, estimate the following:
a. Annual average water demand for the entire subdivision in m3/d
b. Residential, commercial, industrial, and institutional water demands and their percentages with respect to total municipal water demand
c. Annual average wastewater flow if 85% average water supply is returned into the municipal sewers.
Water-Using Establishment Source of Water Demand Residential
Single-family detached residential population 1000 residents
Apartments 1650 residents
Trailer park 400 residents
Commercial
One hotel=motel 250 units
Five restaurants, total seating 300 customers
Shopping centers 250 employees
Office buildings 500 employees
Office complexes 600 m2
One movie theater 200 seats
One commercial laundry 40 machines
Barbershops 10 chairs
Beauty salons 20 stations
One service station 6 bays
Industrial
Factories with shower 100 employee-shifts
Light industrial zone 2 ha
One feedlot 1000 heads
One canning plant 4 metric tons
One meat packaging plant 3 metric tons
Institutional
One nursing home 50 beds
One hospital 200 beds
Two daytime schools 1200 students
Solution
1. Estimate the residential population of the subdivision.
Single-family detached housing residents ¼1000
Apartment residents ¼1650
Trailer park residents ¼400
Total ¼3050 residents
2. Estimate the residential water demand.
Single-family detached housing ¼1000 residents0:315 m3=persond ¼315 m3=d Apartments ¼1650 residents0:205 m3=persond ¼338 m3=d Trailer park ¼400 residents0.15 m3=person·d ¼60 m3=d
Total residential demand ¼713 m3=d
3. Estimate the commercial water demand.
Hotel=motel ¼250 units0:38 m3=unitd ¼95 m3=d
Restaurant ¼300 customers0:03 m3=customerd ¼9 m3=d Shopping centers ¼250 employees0:04 m3=employeed ¼10 m3=d Office building ¼500 employees0:065 m3=employeed ¼32.5 m3=d Office complexes ¼600 m20:015 m3=employeed ¼9 m3=d Movie theater ¼200 seats0:008 m3=seatd ¼1.6 m3=d
Laundromat ¼40 machines2:5 m3=machined ¼100 m3=d
Barber shops ¼10 chairs0:065 m3=chaird ¼0.65 m3=d Beauty salons ¼20 stations1:026 m3=stationd ¼20.5 m3=d Service station
Fist bay ¼1 bay3:8 m3=bayd ¼3.8 m3=d
Additional bays ¼5 bays1:9 m3=bayd ¼9:5 m3=d
Total commercial demand ¼292 m3=d
4. Estimate the industrial water demand.
Factories with shower ¼100 employee-shifts0:133 m3=employee-shift
¼13 m3=d Light industrial zone ¼2 ha11:5 m3=had ¼23 m3=d Feedlot ¼1000 heads0:045 m3=headd ¼45 m3=d Canning plant ¼4 metric tons=d45 m3=metric ton ¼180 m3=d Meat packaging plant ¼3 metric tons=d20 m3=metric ton ¼60 m3=d
Total industrial water demand ¼321 m3=d
5. Estimate the institutional water demand.
Nursing home ¼50 beds0:38 m3=bedd ¼19 m3=d
Hospital ¼100 beds0:95 m3=bedd ¼95 m3=d
School ¼1200 students0:076 m3=studentd ¼91:2 m3=d
Total institutional water demand ¼205 m3=d
6. Estimate the annual municipal water demand and their percentages.
Components Demand Percentage
Residential ¼713 m3=d 47
Commercial ¼292 m3=d 19
Industrial ¼321 m3=d 21
Institutional ¼205 m3=d 13
Total ¼1531 m3=d 100
Average municipal water demand=1531 m3/d×1000 L
m3 × 1
3050 residents=502 Lpcd(133 gpcd) Note: 1 gallon=3.8 L
7. Estimate the annual average wastewaterflow.
Annual wastewater flow =0.85×1531 m3/d=1301 m3/d or 427 Lpcd(112 gpcd)
EXAMPLE 4.5: WATER SAVING FROM SUPPLY PRESSURE REDUCTION
In a community of 50,000 residents, the average water demand is 630 Lpcd and supply pressure is 414 kN/m2. Average water lost through cracks at this pressure is 15%. How much saving would be achieved if the supply pressure is reduced to 276 kN/m2? Assume cracks behave like orifices.
Solution
1. Estimate the average water demand, and average water lost through the cracks at the pressure of 414 kN/m2.
Average water demand ¼50,000 people 630 L
persond m3
1000 L¼31,500 m3=d Water lost through cracks¼0.1531,500 m3=d¼4725 m3=d
2. Select the orifice equation.
Velocity and discharge through an orifice are expressed by Equations 4.2 and 4.3.
V=
2gh
(4.2) q=CdAV=CdA
2gh
(4.3) where
V =velocity through an orifice, m/s q =discharge through an orifice, m3/s
Cd=coefficient of discharge. Typical value for orifice is 0.61.
A =area of orifice, m2
g =acceleration due to gravity, m/s2 h =water head over the orifice, m
3. Calculate the water heads at pressures of 414 and 276 kN/m2.
There is a direct conversion between the pressure head in kN/m2and water head in m based on specific weight of water. At 4◦C (39◦F), the specific weight for water is 9.81 kN/m3(62.4 lb/ft3) and the static head of water in meter is expressed below:
Head in meter=Pressure in kN/m2 9.81 kN/m3
Water head at pressure of 414 kN/m2,h414= 414 kN/m2
9.81 kN/m3=42.2 m Water head at pressure of 276 kN/m2,h276=276 kN/m2
9.81 kN/m3=28.1 m 4. Calculate the loss of water through the cracks at pressures 414 and 276 kN/m2.
q414=CdA 2gh414
q276=CdA
2gh276
q414
q276=CdA 2gh414
CdA
2gh276
= h414
h276
=
42.2 m 28.1 m
=1.23
Water lost through the cracks at 414 kN/m2,q414=4725 m3/d Water lost through the cracks at 276 kN/m2, q276=q414
1.23=4725 m3/d
1.23 =3841 m3/d
5. Calculate the saving.
Saving achieved by reduction of supply pressure from 414 to 276 kN/m2
=(4725−3841) m3/d=884 m3/d or 17.7 Lpcd
Note: The average water loss through the cracks is reduced by about 19% when the supply pressure is lowered. It counts about 2.8% of the average daily water demand.
EXAMPLE 4.6: WATER SAVING FROM PRESSURE REDUCING VALVE (PRV) A home has installed a PRV at the point of entry into the home. The PRV reduces supply pressure from 500 to 300 kN/m2. If water usage before PRV is 350 Lpcd, calculate typical water saving. Use the infor- mation provided inTable 4.2.
Solution
1. Select fromTable 4.2the residential water uses that are affected by PRV.
The toiletflush tank has afixed volume, and the washing machine and dishwasher have level controller. Therefore, installation of the PRV will not reduce water uses in these categories. Only those uses will be affected that are directly from the faucet. These uses are: (a) shower 20% (bathing 8% not affected), (b) wash basin 11%, (c) kitchen assume 4% (out of 9%), and (d) lawn sprinkling 3%.
2. Calculate the water uses in home directly affected by PRV.
Total percentage affected=(20+11+4+3)%=38%
Total usage affected at 500 kN/m2,q500=0.38×350 Lpcd=133 Lpcd 3. Calculate the ratio of usage before and after installing the PRV.
q500
q300=CdA 2g h500
CdA
2g h300
=
P500
9.81 kN/m3
P300
9.81 kN/m3
= P500
P300
=
500 kN/m2 300 kN/m2
=1.29
4. Calculate the water saving due to PRV.
Total usage at 300 kN/m2,q300=q500
1.29=133 Lpcd
1.29 =103 Lpcd Total water saving,Δq=q500−q300=133 Lpcd−103 Lpcd=30 Lpcd Overall water saving based on average water demand= Δq
q500= 30 Lpcd
350 Lpcd×100%=8.6%
EXAMPLE 4.7: WATER CONSERVATION IN HOMES
A home has installed water conservation devices. These devices and expectedpercentage savings over conventional devices are given inTable 4.4. Calculate total water saving in Lpcd and overall percent saving. The average water usage in the home before installation of these devices was 400 Lpcd.